Process for helium enrichment

ABSTRACT

A method is provided for extracting helium from a relatively helium poor gas mixture through a pressure swing adsorption process which adhieves an enriched product gas containing over 50% helium. The gas mixture to be enriched is fed cyclically to four adsorber vessels ranged in parallel which successively pass through a pressure build-up, an adsorptive and a pressure relief phase, with pressure build-up and relief being in part brought about by pressure compensation with any one of the other adsorbers. The pressure-build up and relief phases include a series of three and four, respectively, steps involving alternating pressure levels. As a preliminary treatment, the process may include an initial step wherein the unenriched gas mixture is first fed through a series of pre-filters filled with activated carbon for removing higher hydrocarbons.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Phase application of PCT/EP 88/00441filed 19 May 1988 and based upon German application P 37 16 898.3 filed20 May 1987 under the International Convention.

FIELD OF THE INVENTION

The invention relates to a method of helium enrichment according to apressure swing adsorption process, from gas mixtures containing helium,nitrogen, methane and other gases passed through carbon molecular sieveswhich adsorb nitrogen and methane and the mentioned other gases, whereinthe gas mixture is fed cyclically to four adsorber vessels arranged inparallel, which successively pass through a pressure build-up, anadsorptive and a pressure-relief phase, with pressure build-up andrelief taking place in part through pressure compensation with any oneof the other adsorbers.

BACKGROUND OF THE INVENTION

A pressure swing adsorption process as hereinabove described is knownfrom European patent specification 0 092 695 for the purification ofhelium, wherein starting with a mixture containing helium and consistingessentially of nitrogen, argon and oxygen, as well as smaller fractionsof carbon dioxide and methane, and using molecular sieves, helium with apurity of over 99.9% by volume can be obtained. However, the initialmixture in this process already has a content of 50-95% by volume ofhelium. This process is not suitable for gas mixtures containing only upto 10% helium, since the process cycle and the combination of processsteps are not suitable.

From U.S. Pat. No. 3,636,679 a further pressure swing adsorption processfor helium enrichment is known, wherein the gas mixture is cyclicallyfed to 4 adsorbers, each of them passing successively through a pressurebuild-up phase, an adsorption phase and a pressure relief phase, wherebythe pressure build-up and the pressure relief are partially performedthrough pressure compensation with two different adsorbers, the pressurebuild-up phase having three steps, and the pressure relief phase havingfour steps. However, in this case, besides the pressure relief steps anadditional flushing step is required for a complete regeneration. Theproduct gas yield (according to Example 53%) is therefore comparativelylow. Furthermore, it is disadvantageous that the product gas recoverytakes place at low process pressure (during the pressure relief steps),so that the product gas has to be brought to a higher pressure level formany purposes of use. Moreover, the various pressure levels, at whichthe product is obtained, require a pressure compensation, which againcan lead to oscillations in the amount of product gas.

Helium is increasingly in demand for several applications, e.g.refrigeration plants for refrigeration, as a shielding gas duringwelding and in the chemical industry, as inert gas in space technology,as a respiration gas during diving, as a carrier gas in chromatography,for the detection of leakages, as a balloon-filling gas and for otherpurposes as well. For these purposes, helium is required with a highdegree of purity. In order to achieve this high purity level, in gasmixtures containing only low levels of helium, several process steps arerequired, in order to first enrich the gas mixture with helium and thento recover high purity helium from this helium-enriched gas mixture.

Helium is enriched and recovered mainly from helium-containing naturalgases. The main components of these gases are nitrogen and methane, aswell as up to 10% by vol. helium, besides lower proportions of severalhigher-molecular weight hydrocarbons and carbon dioxide.

According to the state of the art, the following method of heliumenrichment is known, as has been published in "Bureau of Mines, Preprintfrom Bulletin 675--Helium--1985 Edition, U.S. Department of Interior",pages 3 and 4.

The helium-containing natural gas is cooled down to approx. -150° in acryogenation plant, whereby primarily the hydrocarbons will be releasedby condensation. The so-produced gas mixture, except for low proportionsof other gases contains more than 50% by vol. helium and nitrogen. Suchcrude helium can be treated on the spot to give a helium of very highpurity, e.g. by subjecting it to some process combination comprising apressure swing adsorption plant plus a second cryogenation unit.

Another alternative is to sell the crude helium as an intermediateproduct to be treated by some third party.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to perform heliumenrichment from natural gases low in helium, subjecting them to pressureswing adsorption only, to give a high yield in crude helium of a puritypossibly over 50%, without the need of intermediate enrichment incryogenation plants.

These requirements are met by a process of the above-mentioned type andnamely through the following procedure involving:

a/ A pressure build-up phase which comprises three steps:

1. Pressure increase from a final vacuum pressure (P₁) to a mediumpressure level (P₃);

2. Pressure increase from the medium pressure level (P₃) to a higherpressure level (P₄);

3. Pressure increase from the higher pressure level (P₄) to the highestpressure level (P₅)--adsorption pressure;

b/ A pressure relief phase which comprises four steps:

1. Pressure relief from the highest pressure level (P₅) down to thehigher pressure level (P₄);

2. Pressure relief from the higher pressure level (P₄) down to themedium pressure level (P₃);

3. Pressure relief from the medium pressure level (P₃) down to theambient pressure (P₂);

4. Pressure relief from the ambient pressure (P₂) down to the finalvacuum pressure (P₁);

c/ Pressure compensation brought about in two steps with the firstcompensation step taking place between the outlet of a first adsorberwhere the first pressure relief step (from P₅ down to P₄) occurs and anoutlet of a second adsorber where the second pressure build-up step(from P₃ up to P₄) occurs, and with the second compensation step takingplace between an outlet of the first adsorber where the second pressurerelief step (from P₄ down to P₃) occurs and an inlet of a third adsorberwhere the first pressure build-up step (from P₁ up to P₃) occurs; and

d/ A third as well as a fourth pressure relief step are effected bycounterflow, whereby a waste gas low in helium is obtained and the thirdpressure increase step is effected with product gas.

As adsorbents for the process of the invention, carbon molecular sievesare used, with an average adsorptive pore diameter between 0.1 and 0.4nm, preferably between 0.3 and 0.4 nm. These sieves are extremelyeffective in separating nitrogen and methane from helium. Surprisinglyalready after one single step, a helium of comparably high purity (over50%) with a high helium yield of more than 90% is obtained whenproceeding according to the suggested teaching. This will be attainablewith initial gas mixtures of comparably low helium concentrationsbetween approx. 8 and 2%, exclusively by the application of pressureswing technology which no longer requires any cryogenation plant, i.e.with very low energy consumption.

It is advisable to provide adsorptive prefilters filled with activatedcarbon, in order to release higher-molecular hydrocarbons and tracecontamination.

Tests have shown that the highest pressure level (P₅), i.e. adsorptionpressure, should be over 1 bar, preferably 10-30 bar, and the finalvacuum pressure below 500 mbar, preferably 50 mbar.

According to a preferred embodiment, to the individual pressure stepsthe following preferential pressure levels are associated:

P₁ =50 mbar

P₂ =1 bar

P₃ =4 bar

P₄ =11.7 bar

P₅ =20 bar

The entire operational cycle lasts between 460 and 3600 s., preferably720 s.

According to a preferred embodiment, in an operational cycle lasting 720s, the pressure relief phase can comprise the following time intervals:

    ______________________________________                                        1.    Pressure relief step from P.sub.5 down to P.sub.4                                                      55     s                                             Rest position            115    s                                       2.    Pressure relief step from P.sub.4 down to P.sub.3                                                      10     s                                       3.    Pressure relief step from P.sub.3 down to P.sub.2                                                      55     s                                       4.    Pressure relief step from P.sub.2 down to P.sub.1                                                      115    s                                       ______________________________________                                    

Also according to a preferred embodiment, the pressure build-up phase ofthe operational cycle lasting 720 s is suitably subdivided into thefollowing time intervals:

    ______________________________________                                        1.    Pressure build-up step from P.sub.1 to P.sub.3                                                         10     s                                       2.    Pressure build-up step from P.sub.3 to P.sub.4                                                         55     s                                       3.    Pressure build-up step from P.sub.4 to P.sub.5                                                         125    s                                       ______________________________________                                    

In an operational cycle lasting 720 s it is advisable to select as atime interval from the product gas recovery 180 s.

The process of the invention is particularly suitable for the enrichmentof helium from feed gases whose helium content is of 10% by vol. orlower, preferably 2-8% by volume, whereby the helium content in therecovered crude helium is of up to 95% by volume.

The process of the present invention is preferred in the heliumenrichment from natural gases, which after a prior separation fromhigher-molecular hydrocarbons and trace contaminations, in adsorptiveprefilters known per se, can have the following composition (given in %by volume):

    ______________________________________                                               N.sub.2                                                                            40-80                                                                    He   2-8                                                                      CH.sub.4                                                                           10-40                                                                    CO.sub.2                                                                           <0,1-5                                                            ______________________________________                                    

BRIEF DESCRIPTION OF THE DRAWING

Further advantages and embodiments can be taken from the description ofpractical examples given hereafter in connection with the encloseddrawings where:

FIG. 1 is a single-step helium recovery unit including four adsorbersarranged in parallel, for helium enrichment up to 95% by vol. purity;

FIG. 2 is a pressure/time diagram of an adsorber within the plant,according to FIG. 1;

FIG. 3 is a pressure/time diagram and table of process incrementsachieved through operation of the four plant adsorbers according to FIG.1;

FIG. 4 is a valve circuit diagram for the four plant adsorbers thehelium purity in a plant according to FIG. 1.

FIG. 5 is a diagram showing the dependency of helium yield on the heliumpurity in a plant (see FIG. 1).

DETAILED DESCRIPTION

The plant as shown in FIG. 1 consists of four adsorbers A through D,arranged in parallel, filled with carbon molecular sieve and, as thecase may be, of four prefilters F 1 through F 4 filled with activatedcarbon and where, if need be, any higher-molecular hydrocarbons andtrace contaminents can be removed from the feed gas prior to itsentering the adsorbers A through D. Each adsorber passes the followingeight steps in successive cycles and in a staggered time pattern inrelation to the remaining three adsorbers:

T1--Adsorption

T2--Pressure relief by compensation (Da 1)

T3--Pressure relief by compensation (Da 2)

T4--Counter-flow pressure relief (GEE)

T5--Evacuation (Ev)

T6--Pressure build-up by compensation (DA 1)

T7--Pressure build-up by compensation (DA 2)

T8--Pressure build-up using product gas (DA 3)

Before dealing with the details of FIG. 1, we shall first describe thesequence of the eight incremental steps T1 through T8 set forth on thepressure/time profiles represented in FIGS. 2 and 3.

FIG. 2 shows the typical pressure/time profile for 20 bar adsorptionpressure and 720 s overall cycle duration. The profile applies to eachof the four adsorbers, with a given staggering in time. On the pressureaxis have been plotted the five pressure levels from P1 through P5between which in our case will take place the pressure build-up, viz.relief steps.

FIG. 3 shows the pressure/time profiles, staggered in time, within thefour adsorbers A through D. Hereafter will be described the typicaloperational cycles going on in adsorber A; identical cycles will bevalid for the remaining three adsorbers B, C and D.

Adsorption (step T1) takes place at a constant, increased pressurelevel, e.g. at 20 bar. At such pressure adsorber A is traversed by thefeed gas during which action nitrogen, methane and other gas componantsare adsorbed by the carbon molecular sieve so that helium which resiststo adsorption leaves the adsorber with a high degree of purity.

After an adsorption cycle the loaded adsorber A is subjected to severalpressure relief steps (T2 through T5) for regeneration.

Such regeneration starts by a first pressure compensation Da 1 (step T2)during which the gas, being at adsorptive pressure, passes in a parallelflow from adsorber A to adsorber C to be brought down from pressurelevel P₅ to the lower level of P₃. The passage of the gas from adsorberA (T2) to adsorber C(T7) has been identified by an arrow in the sequenceof process steps as per FIG. 3.

During the first pressure compensation step (Da 1) the pressure inadsorber A is relieved to pressure level P₄ (which may be 11.7 barwhereas the pressure prevailing in adsorber C increases simultaneouslyfrom level P₃ to level P₄ (pressure build-up DA 2).

After a short rest position (standby), a second pressure compensation(DA 2, step T3) takes place in adsorber A when the gas, being atpressure level P₄, leaves adsorber A (again in a parallel flow) to bepassed for pressure relief to the adsorber D which is at vacuum pressureP₁. All the while the pressure P₄ prevailing in adsorber A drops tolevel P₃ which may be 4 bar. During both of the pressure compensationsteps a helium-enriched gas mixture will flow from adsorber A toadsorber C, viz. to adsorber D.

Following the two pressure compensation steps (Da 1 and Da 2) adsorber Awill, by counter-flow, continue to be relieved, this time from level P₃to ambient pressure P₂ (GEE, step T4). All the while a gas mixture lowin helium is yielded which is rather high in nitrogen and methane andother components having been desorbed during counter-flow relief (GEE);this gas mixture is discarded.

Hereafter adsorber A will be evacuated by vacuum pump 80 to a vacuumpressure P₁ of e.g. 50 mbar (Ev, step T5). During this procedurenitrogen and methane as well as other gas component having been adsorbedduring the prior step T1 will increasingly become desorbed. Theevacuated gas is extremely low in helium and therefore discarded aswell.

With the evacuation step, regeneration of adsorber A is completed wherenow the pressure will gradually be built up by steps T6 through T8 untilhaving arrived at adsorption pressure P₅.

The first pressure compensation (step T6) takes place between adsorber Awill, by counter-flow, continue to be relieved, T2 and is--evacuation ofadsorber A nearing completion--at a higher intermediate pressure P₄.During pressure compensation, a helium-enriched gas mixture flows fromadsorber B to adsorber A, this mixture being withdrawn from adsorber Bpreferably in a parallel flow to enter adsorber A likewise in a parallelflow (top/bottom pressure compensation). All the while pressure inadsorber A (build-up DA 1) rises from the final vacuum pressure P₁ to anintermediate level P₃ which may be at 4 bar, whereas simultaneously theintermediate pressure level P₄ prevailing in adsorber B will drop to thelower intermediate level P₃.

The following pressure compensation (step T7) with adsorber C help tofurther increase the pressure in adsorber A (build-up DA 2). Prior tothat compensation, adsorber C has passed through step T1 (adsorption)and is on the point of being pressure-compensated with adsorber A whichlatter is at the adsorption pressure level P₅. In the example quoted,pressure compensation is done in such a way that the helium-enriched gasmixture is withdrawn in a parallel flow from adsorber C to be relievedby entering adsorber A in a counter-flow (top/top pressurecompensation). All the while the pressure in adsorber A rises from anintermediate level P₃ to the nextmost level P₄ which may be at 11.5 bar,whereas at the same time the adsorption pressure P₅ prevailing inadsorber C drops to the intermediate level P₄.

After the aforesaid double pressure compensation, product gas is used toincrease the pressure in adsorber A from the higher intermediate levelP₄ to the adsorption pressure level P₅ (e.g. 20 bar) (build-up DA 3,step T8), after which will start another adsorption step in adsorber A(step T1).

As shown in FIG. 1 the four adsorbers A through D are switched via anumber of valves in such a way that one of the four is always atadsorption pressure to produce helium of high purity as product gas.FIG. 4 represents the switching circuit of the valves. The following isto explain, based on FIGS. 4 and 1 and taking adsorber A as an example,the supply and abduction of gases in the pressure swing adsorption plantas represented on FIG. 1. The adsorbers A through D may be preceded byprefilters F1 through F4 which are a technical standard and serve forthe preliminary removal of highly adsorbing gas constituents ofhigher-molecular hydrocarbons from oil well gases. As shown by theexample, filter operation normally is similar to that of the filters atthe subsequent main adsorbers A through D arranged in series andtherefore these further operations are not discussed.

Upon pressure build-up by product gas (DA 3, step T8), adsorber A is atadsorption pressure P₅. During subsequent adsorption (step T1) feed gasarrives through line 1 at a pressure which, for overcoming the pressuredrop within the plant, is slightly above adsorption pressure, with openvalves 10 and 13 arranged in flow direction, upstream, viz. behindadsorber A, and flows through adsorber A. With the exception of heliumall the other constituents of the feed gas, such as nitrogen andmethane, will be adsorbed thereby on the carbon molecular sieve so thata helium-enriched gas leaves the top of adsorber A via line 4 and needlevalve 71 (adjusting valve) to be discharged through a product gas line91. The adsorption is subdivided in three time steps Z1, Z2 and Z3matching with the valve circuit diagram on FIG. 4. During step Z1 avalve 50 arranged in line 5 is closed so that all of the product gasenters the product gas line 91 via line 4. During steps Z2 and Z3 valve50 opens so that part of the product gas enters the adsorber B via asubsequent choke 72, via line 5 and the open valve 25 preceding adsorberB which latter, under the action of the product gas having enteredduring step T8, is boosted from the intermediate pressure level P₄ toadsorption pressure P₅. The duration of said time steps Z1, Z2 and Z3may, with an overall cycle duration of 720 s, amount to 55 s for stepZ1, 115 s for step Z2 and 10 s for step Z3.

Adsorber A will, after its adsorptive phase, be pressure-relieved bystep T2 (Da 1) to the higher intermediate level P₄ during which the gasdischarged from adsorber A whose valve 15 has opened (while valve 50 isclosed) arrives for top/top pressure compensation via choke 73 in line5, with open valve 35, at adsorber C which, in turn, is now subjected tostep T7 whereby it is boosted from intermediate pressure P₃ to a higherintermediate level P₄. According to the valve circuit diagram on FIG. 4such pressure compensation (Da 1) takes the time step Z1 which in ourexample lasts for 55 s, with an overall cycle of 720 s.

Upon the aforesaid first pressure compensation and rest position(standby) which latter, for an overall cycle of 720 s, accounts for 115s, adsorber A is further relieved by step T3 (Da 2) via another pressurecompensation with adsorber D, i.e. brought down from the higherintermediate level P₄ to the lower intermediate level P₃. To this endgas from absorber A is led, with valves 14 and 42 opened, via a ringline 3 (valve 60 in line 92 closed) and via choke 74, to adsorber Dwhich latter, in turn, is being subjected to step T6 and boosted fromits final vacuum pressure P₁ to the intermediate level P₃. The pressurecompensation in our case happens therefore in a top/bottom mode.According to the valve circuit diagram in FIG. 4, the pressurecompensation Da 2 lasts for the time span Z3 which in the quoted exampletakes 10 s of an overall cycle of 720 s.

Hereafter the pressure prevailing in adsorber A is further reduced bystep T4 (GEE) in a counter-flow, with opened valves 12 and 60, via choke75, from the intermediate level P₃ down to ambient pressure P₂. The gasdischarged during this enters a waste gas line 92. In our example said,the counter-flow pressure relief lasts 55 s on a total cycle of 720 s.

Upon counter-flow pressure relief adsorber A will be evacuated by stepT5 (Ev), with opened valve 11, by means of vacuum pump 80, from ambientpressure P₂ until having arrived at a final vacuum pressure P₁ which maybe 50 mbar. The gas mixture low in helium withdrawn during evacuationenters waste gas line 93. In our example evacuation takes 115 s on anoverall cycle of 720 s.

Thereafter the evacuated adsorber A is, by step T6 (DA 1), brought fromits final vacuum pressure P₁ to intermediate pressure P₃ in acompensation with adsorber B. The step is effected preferably astop/bottom pressure compensation. During this step, a helium-enrichedgas mixture is pressure-relieved by passing from the outlet of adsorberB, with the valves 24 and 12 open (valve 60 being closed), via ring line3 and choke 74 to the inlet of adsorber A. All the while adsorber Bpasses through step T3. During pressure compensation the pressure inadsorber B drops from an intermediate level P₄ to the lower intermediatelevel P₃. Pressure compensation DA 1 takes 10 s on an overall cycle of720 s duration.

Adsorber A, now boosted to the intermediate pressure level P₃, ishereafter subjected to further pressure build-up by step T7 (DA 2)bringing it to intermediate level P₄ in another pressure compensationwith adsorber C. This compensation is carried out preferably in thetop/top mode so that a helium-enriched gas mixture becomes destressedpassing from the outlet of adsorber C with open valves 35 and 15 viachoke 73 in line 5, to the outlet of adsorber A. All the while adsorberC is subjected to step T2 whereby pressure in adsorber C drops fromadsorption pressure P₅ to an intermediate level P₄. Pressurecompensation DA 2 takes 55 s on an entire cycle of 720 s duration.

Finally adsorber A is boosted by step T8 (DA 3) using product gas tobring it from the intermediate level P₄ to adsorption pressure P₅. Tothis end, part of the product gas is passed to adsorber A via choke 72and open valves 50 and 15. According to the valve circuit diagram onFIG. 4 pressure build-up DA 3 is composed of the 2 time steps Z2 and Z3taking 115 viz. 10 s on an overall cycle of 720 s.

Upon pressure build-up DA 3 using product gas another pressure swingcycle starts in adsorber A, commencing again by the adsorption step. Thepressure swing cycle in the adsorbers B, C and D runs accordingly,although staggered in time, as can be taken from FIG. 3. Respectivevalve arrangements 20, 21, 22, 23, 30, 31, 32, 33, 34, 40, 41, 43, 44and 45 for adsorbers B, C and D operate in similar sequence to thoseassociated with adsorber A as hereinabove described and illustrated inFIG. 4.

As was described earlier, regeneration of the adsorbent is brought aboutby an evacuation step although with the present state of the art the gasconstituents, such as nitrogen and methane, to be removed from thehelium-containing feedgas could be gotten rid of by product gasflushing. Such flushing desorption would, however, lead to unacceptablyhigh helium yield losses in the case of helium recovery from naturalgases since given the low helium content in the feedgas the product gasyield in the form of high purity helium gas is modest, with big gasvolumes having to be desorbed at the same time, the gas constituentshaving to be removed by adsorption and re-desorbed again account for atleast 90% by vol. of the feed gas.

EXAMPLES

In a pressure swing plant on a laboratory scale, according to FIG. 1(however excluding the prefilters F1 through F4), applying an adsorptionpressure of 20 bar, a final vacuum pressure of 50 mbar and an overallcycle duration of 720 s corresponding to 5 cycles/h, separationexperiments were carried out using a gas mixture containing helium(about 5% by vol.), methane (about 29% by vol.) and nitrogen (about 66%by vol.). The four adsorbers A through D were filled with carbonmolecular sieve of an average adsorptive pore diameter of 0.35 nm; theirfilling capacity amounted to 2 l/adsorber. During the experiments thevolume of product gas was adjusted by setting of the needle valve 71whereby the degree of helium purity in the product gas was varied at thesame time. The test results summarized in tables 1 through 4 belowsupport the doctrine of the invention that helium can, indeed, beenriched to a helium purity between 75 and 95% by vol. in the productgas, starting from a feedgas containing <10% by vol. helium, with,depending on the helium purity in the product gas, a helium yieldbetween 90-99.9% being attained. The experimental results are laid downin the form of a complete mass balance.

                  TABLE 1                                                         ______________________________________                                                     Concentration                                                                 (% by vol.)  Volume                                                           He   CH.sub.4 N.sub.2                                                                              (Nl/h)                                      ______________________________________                                        Feedgas        5,1    28,9     66,0 602,2                                     Evacuation waste gas                                                                         0,7    23,1     76,2 191,3                                     Waste gas from 0,5    34,0     65,5 381,7                                     counter-flow relief                                                           Product gas    95,0   --        5,0 29,                                       ______________________________________                                    

From the above can be computed a helium yield of 90.3%.

                  TABLE 2                                                         ______________________________________                                                     Concentration                                                                 (% by vol.)  Volume                                                           He   CH.sub.4 N.sub.2                                                                              (Nl/h)                                      ______________________________________                                        Feedgas        5,3    28,9     65,8 593,6                                     Evacuation waste gas                                                                         0,2    23,8     76,0 188,5                                     Waste gas from 0,2    34,1     65,7 371,5                                     counter-flow relief                                                           Product gas    90,0   --       10,0  33,6                                     (crude helium)                                                                ______________________________________                                    

From the above can be computed a helium yield of 96,1%.

                  TABLE 3                                                         ______________________________________                                                     Concentration                                                                 (% by vol.)  Volume                                                           He   CH.sub.4 N.sub.2                                                                              (Nl/h)                                      ______________________________________                                        Feedgas         5,1   28,6     66,3 604,1                                     Evacuation waste gas                                                                         --     24,5     75,5 190,4                                     Waste gas from <0,1   33,7     66,2 375,2                                     counter-flow relief                                                           Product gas     80,0  --       20,0  38,5                                     (crude helium)                                                                ______________________________________                                    

From the above can be computed a helium yield of 99.9%.

                  TABLE 4                                                         ______________________________________                                                     Concentration                                                                 (% by vol.)  Volume                                                           He   CH.sub.4 N.sub.2                                                                              (Nl/h)                                      ______________________________________                                        Feedgas         5,4   28,5     66,1 609,4                                     Evacuation waste gas                                                                         --     26,0     74,0 194,1                                     Waste gas from <0,1   33,1     66,8 372,4                                     counter-flow relief                                                           Product gas     76,4  --       23,6  42,9                                     (crude helium)                                                                ______________________________________                                    

From the above can be computed a helium yield of 99.6%.

With increasing helium purity, (helium content of the recovered crudehelium) the helium yield decreases and vice versa. The interdependencebetween helium purity (helium content of the recovered crude helium) andthe helium yield is represented in FIG. 5.

We claim:
 1. Method of helium enrichment, according to a pressure swingadsorption process, from a gas mixture comprising helium, nitrogen andmethane, passed through carbon molecular sieves which adsorb nitrogenand methane where the gas mixture is fed cyclically to four adsorbervessels arranged in parallel which successively pass through a pressurebuild-up, an adsorptive and a pressure relief phase, with pressurebuild-up and relief being in part brought about by pressure compensationwith anyone of the other adsorbers wherein;a) the pressure build-upphase comprises the three steps of:1. pressure increase from a finalvacuum pressure (P₁) to a medium pressure (P₃);
 2. pressure increasefrom the medium pressure (P₃) to a higher pressure (P₄);
 3. pressureincrease from the higher level (P₄) to a highest pressure level (P₅)representing an adsorption pressure; b) the pressure relief phasecomprises the four steps of:
 1. pressure relief from the highest level(P₅) down to the higher level (P₄);2. pressure relief from the higherlevel (P₄) down to the medium level (P₃);
 3. pressure relief from themedium level (P₃) down to an ambient pressure (P₂);
 4. pressure relieffrom the ambient pressure (P₂) down to the final vacuum pressure (P₁);c) pressure compensation is brought about in two compensation steps withthe first compensation step taking place between an outlet of a secondadsorber where the second pressure build-up step (from P₃ to P₄) happensand with the second compensation step taking place between an outlet ofa first adsorber where the second pressure relief step (from P₄ down toP₃) happens and an inlet of a third adsorber where the first pressurebuild-up step (from P₁ to P₃) happens and d) the third as well as thefourth pressure relief steps are effected by counter-flow whereby awaste gas low in helium is yielded, whereas the third pressure increaseis effected using product gas.
 2. The method according to claim 1wherein the gas mixture further comprises higher hydrocarbons andwherein the adsorbers are preceded by pre-filters filled with activatedcarbon for removing the higher hydrocarbons from the gas mixture.
 3. Themethod according to claim 1 wherein the highest pressure level (P₅) isgreater than 1 bar and the final vacuum pressure is below 500 mbar. 4.The method according to claim 3 wherein the highest pressure level (P₅)is between 10 and 30 bar and the final vacuum pressure is 50 mbar. 5.The method according to claim 3 wherein pressures are as follows:P₁ =50mbar P₂ =1 bar P₃ =4 bar P₄ =11.7 bar P₅ =20 bar.
 6. The methodaccording to claim 1 wherein an entire operational cycle lasts between415 and 3600 s.
 7. The method according to claim 1 wherein an entireoperational cycle lasts 720 s.
 8. The method according to claim 1wherein the pressure relief phase in an operational cycle lasting 720 scomprises the following time intervals:

    ______________________________________                                       
 1.    Pressure relief step from P.sub.5 down to P.sub.4                                                     55     s                                              Rest position           115    s                                       
 2.    Pressure relief step from P.sub.4 down to P.sub.3                                                     10     s                                       
 3.    Pressure relief step from P.sub.3 down to P.sub.2                                                     55     s                                       
 2.    Pressure relief step from P.sub.2 down to P.sub.1                                                     115    s.                                       ______________________________________                                    


9. The method according to claim 1 wherein the pressure build-up phasein an operational cycle lasting 720 s comprises the following timeintervals:

    ______________________________________                                       
 1.    Pressure build-up step from P.sub.1 to P.sub.3                                                        10     s                                       
 2.    Pressure build-up step from P.sub.3 to P.sub.4                                                        55     s                                       
 3.    Pressure build-up step from P.sub.4 to P.sub.5                                                        125    s.                                       ______________________________________                                    


10. The method according to claim 1 wherein recovery of the product gasextends over a time interval of 180 s in an operational cycle lasting720 s.
 11. The method according to claim 1 wherein the helium proportionin the gas mixture amounts to 10% by volume or less.
 12. The methodaccording to claim 1 wherein the helium proportion in the gas mixtureamounts to between 2 and 8% by volume.
 13. The method according to claim1 wherein natural gas is employed as the gas mixture.